Black Cats duo sidelined
By Graeme Bailey
Last Updated: 01/01/70 1:00am
Thornton suffered ankle ligament damage in a reserve team outing, while Piper damaged knee cartilage in training.
Piper is expected to be out for about eight weeks, while Thornton will be absent for at least a month.
"We think he's damaged his ankle ligaments," said Sunderland boss Mick McCarthy on Thornton.
"We don't think it's as serious as we first thought, but until we can assess it and have X-rays or scans on it and find out exactly the extent of the damage, we won't know.
"Hopefully it's not as bad as was first thought, but it's never nice to see somebody carried off, that's for sure."
On Piper he added: "We've had it looked at and we've seen on the scans that he's actually got a tear in his cartilage, which is very sad because he'd be a welcome addition to the squad.
"People say it's like having a new player - he would be like having a new player.
"It's dreadful luck, absolutely dreadful."